In joining lengths of sheet metal duct butt to butt, opposed pairs of hook-flanges, called drive edges and fashioned on the ends of the ducts, are connected with drives. A drive is a pre-formed sheet metal strap with the long margins symmetrically folded in plane with each other to form an exaggeratedly thin flattened "C"-section. In a typical tight-fit application, just enough space or slot remains between the folded-over margins of the "C"-section to permit uniting the drive edges by sliding the drive along them from one end to the other, sometimes by hammering on a bend end of the drive in the manner of driving a stake. Any space between the ends of the pieces of duct is usually eliminated by the drawing together of the ends by the drive.
To permit one of the drive ends to be bent over after installation so that the drive is held in place at the ducts regardless of vibration or accidental impact, the ends of the folded-over margins are trimmed back with sheet metal snips to leave a single thickness or soft nose an inch or so long at the end, and the corners of this soft-nose are snipped to leave a blunt wedge shape.
To assure easier engagement of the folded over margins with the drive edges, an open throat is formed at the snipped-back ends of the margins by snipping off the corners at the ends of the folded over margins.
Both procedures take time and both cause injury to sheet metal workers from the sharp edges.